1. Field of the Invention
The present invention relates generally to primary well cementing, and more particularly, to methods of removing drilling fluid deposits from the walls of a well bore prior to placing cement therein.
2. Description of the Prior Art
A variety of drilling fluids are used in drilling well bores. Generally, the drilling fluids are solids-containing water base gels or hydrocarbon base fluids which can contain particulate weighting material such as barite. When a well bore is being drilled a drilling fluid is circulated through the well bore; when the drilling is completed, the circulation of the drilling fluid is stopped, the well is logged and pipe is run into the well bore. During the period when no circulation occurs, filter cake and partially dehydrated gelled drilling fluid are deposited on the walls of the well bore. The partially dehydrated gelled drilling fluid is formed from drilling fluid adjacent the walls of the well bore which loses a portion of its water and which develops gel strength in the absence of shear caused by the lack of circulation. Drilling fluid adjacent the partially dehydrated drilling fluid also develops gel strength in the absence of shear caused by the lack of circulation.
After pipe is run in the well bore, primary cementing operations are performed wherein the pipe is cemented in the well bore by placing a cement slurry in the annulus between the pipe and the walls of the well bore. When the cement slurry is run into the annulus, drilling fluid is displaced from the well bore. The cement slurry sets into a hard impermeable mass whereby the annulus is sealed.
In order for a primary cementing operation to be successful, it is preferred that all of the gelled drilling fluid and at least a major portion of the partially dehydrated gelled drilling fluid and filter cake deposited on the walls of the well bore be removed. If too much drilling fluid and filter cake deposits remain on the walls of the well bore, the cement will not properly bond thereto and fluid leakage through the well bore and other major problems will result.
Heretofore, attempts have been made to condition the drilling fluid in the well bore after logging and running pipe by circulating the drilling fluid through the well bore while removing drilling solids therefrom. It is believed that such circulation causes the erosion and thus facilitates the removal of a major portion of the gelled and partially dehydrated gelled drilling fluid and filter cake from the walls of the well bore. To estimate the quantity of material removed, attempts have been made to measure the circulating volume of drilling fluid in the well bore and then, by comparing that measurement with the total volume available in the well bore for drilling fluid, an estimate of the drilling fluid deposits remaining on the walls of the well bore is obtained.
Prior art techniques for determining the volume of circulating drilling fluid have involved combining marker fluids or materials with the circulating drilling fluid at the surface and measuring the time required for the marker to flow through the well bore and reappear at the surface. The time has been multiplied by the pumping rate of the drilling fluid being circulated to estimate the circulating volume. Examples of marking fluids and materials which have been used are oil based paints, calcium carbide pills, lightweight solids and the like. The use of such marker fluids and materials is very cumbersome and does not provide an accurate measurement of the circulating volume.
Thus, there is a need for improved methods of determining the circulating volume of a drilling fluid in a well bore and for maximizing the removal of drilling fluid deposits from the walls of the well bore prior to cementing.